A stable plasma requires certain physical conditions to exist. However, even when those conditions exist, a plasma may not spontaneously ignite. Examples of that phenomenon are well-known; for example, atmospheric arc welders require an ‘RF start’. Another known plasma-ignition technique involves the induction of a ‘spark’ igniter using a Tesla coil. However, both of those techniques involve the use of metallic components in the plasma reaction chamber, which can be disadvantageous. In the case of microwave-pumped systems, such metal components are found to ‘ground’ the plasma and cause it to be unstable.
Other known methods used to ignite a plasma include reducing the pressure of the gas from which the plasma is to be formed and the introduction of argon, helium or some other gas or gases that are more easily ionised than the principal plasma gas.
It is particularly important to provide reliable ignition in a microwave plasma. In forming such a plasma, microwaves are generally provided by a magnetron and are transmitted along a waveguide to the plasma, where their energy is absorbed by the plasma, typically in a standing-wave arrangement. However, if the plasma is not ignited (i.e. if there is no plasma but only gas) then little energy is absorbed and, in the standing-wave arrangement, a significant amount of the incident energy is reflected back to the magnetron, which can severely shorten its lifetime. Such back-reflections may be reduced by including a one-way circulator or ‘valve’ in the microwave transmission line but such an arrangement adds to the cost of the device. A method of reliably igniting a microwave plasma is therefore desirable.
Plasma abatement has become a widely used method of eliminating exhaust gases from manufacturing processes, and is of particular application in the degradation of perhalogenated compounds especially perfluorinated compounds (PFCs).
PFCs are commonly used in the semiconductor manufacturing industry, for example, in dielectric film etching, and following the manufacturing process there is typically a residual PFC content in effluent gases. The PFCs are difficult to remove from the effluent. Their release into the environment is undesirable because they are known to have relatively high greenhouse activity. A variety of abatement methods have been used previously, for example, combustion, reactive adsorption and catalytic oxidation. The objective of abatement is to convert the PFC into one or more compounds that can be more conveniently disposed of, for example, by conventional scrubbing.
Plasma abatement has proved to be an effective method for degradation of PFCs to less damaging species. In the plasma abatement process, an effluent gas containing the species to be destroyed is caused to flow into a high density plasma and under the intensive conditions within the plasma the PFCs are subjected to impact with energetic electrons causing dissociation into reactive species which can combine with oxygen or hydrogen to produce relatively stable, low molecular weight by-products, for example, CO, CO2 and HF, which can then be removed in a further treatment step.
In one form of previously known plasma abatement, the plasma is a microwave plasma. It is also known to use a radio-frequency plasma.
One form of device suitable for use in microwave plasma abatement is described in UK Patent Specification GB 2273027A. In that device, the microwave plasma is generated between two electrodes, which are in closely opposed relationship. The arrangement shown in GB 2273027A is self-starting. The arrangement of GB 2273027A suffers from a relatively high degree of corrosion of the electrodes by the reaction products.
US2002/0101162 describes a microwave plasma generator that is ignited by a spark from a Tesla coil.